A Computational Study of the Oxidation of the Phenanthryl Radical C$_{14}$H$_9$•: Thermochemistry and Possible Reaction Pathways

In this computational study, the reaction of the phenanthryl radical C$_{14}$H$_9$• (A$_3$•) with molecular and atomic oxygen as part of the reaction mechanism of the oxidation of phenanthrene is investigated. Pathways resulting from the A$_3$• + $^3$O$_2$ and A$_3$• + O reaction systems are examined using DFT quantum chemistry calculations. The energetics of the involved intermediates, products, and transition state structures are determined, as well as the kinetics of possible reaction pathways. Similar to the oxidation of smaller systems (A$_1$• (phenyl) or A$_2$• (naphthyl)), the addition of $^3$O$_2$ to the phenanthryl radical A$_3$• results in a peroxy radical C$_{14}$H$_9$OO• (A3OO•) with some 50 kcal mol$^{−1}$ of energy released, allowing further propagation and/or chain branching reactions. Standard enthalpies of formation as well as entropies and heat capacities are calculated using DFT methods (B3LYP, M06, APFD). Similar to the oxidation of the naphthyl radical, six primary subsequent pathways for the stabilized peroxy A$_3$OO• are investigated with barriers below the energy of the starting point A$_3$• + $^3$O$_2$. The reaction channels were investigated down to the next smaller PAH-radical (naphthyl A$_2$•) and the corresponding reaction products. ... mehrImportant exothermic chain branching reactions are taking place, and several unsaturated oxygenated hydrocarbon intermediates are identified and determined. For the identified pathways, kinetic parameters based on canonical transition state theory (CTST) are calculated and reported in this study.